Bupropion (its salt hydrochloride) is a widely used antidepressant drug. A commercial example is Wellbutrin.RTM.. This product consists in immediate release tablet, 75 or 100 mg strength. However it has been proven that bupropion hydrochloride can induce some severe side effects. For example, seizures can occur in about 0.4% patients and this effect has been proven to be related with immediate release tablets because of the peak in bupropion plasmatic concentration induced by such a dosage form. The development of a new sustained release dosage form has therefore been considered as an appropriate means to overcome this situation.
U.S. Pat. No. 5,358,970 and U.S. Pat. No. 5,427,798, both to Burroughs Wellcome, describe a sustained release formulation of bupropion hydrochloride based on matrix technology. The term matrix refers to a tablet where the drug is embedded in an excipient that makes a non-disintegrating core called matrix. Drug diffusion occurs through this core. As bupropion hydrochloride is unstable, the product described in the above two patents requires a stabilizer to achieve sufficient stability. This stabilizer is an acidic compound, preferably cysteine hydrochloride. Matrix technology is however not suited for the manufacture of a tablet, since it implies the use of a stabilizer.
U.S. Pat. No. 4,687,660 and EP-A-0171457 disclose a tablet formed of a core and a coating, where the core comprises bupropion hydrochloride together with excipient(s) and optionally an osmotic enhancing agent and where the coating comprises a water-insoluble, water-permeable film-forming polymer (such as cellulose acetate), a pore-forming agent (such as impalpable lactose and sodium carbonate), and optionally a so-called water-permeability enhancing agent (such as polyethyleneglycol) and again optionally a plasticizer. This type of coating, since it requires pore-forming agent, cannot provide a uniform coating and therefore the release rate cannot be uniform from one tablet to another.
The prior art thus cannot afford tablets of bupropion hydrochloride without recourse to the matrix technology (and to a stabilizer) or without recourse to pore-forming agent in the coating.